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Building and Environment 39 (2004) 635–643www.elsevier.com/locate/buildenv

Car park ventilation system: performance evaluation

M.Y. Chan∗, W.K. ChowDepartment of Building Services Engineering, The Hong Kong Polytechnic University, Hong Kong, China

Received 22 June 2003; received in revised form 20 September 2003; accepted 14 October 2003

Abstract

Unsatisfactory design of mechanical ventilation systems in car parks would give poor indoor environment. Site investigations on theventilation systems in six car parks were carried out. Physical con5gurations of the car park and ventilation system installed were studied.Carbon monoxide levels were measured during the peak hours. From the results, parameters including the removal e7ectiveness and localair quality index were examined in conjunction with the measured carbon monoxide levels. Performance of di7erent types of ventilationsystems was compared. It is found that the combined supply and exhaust system performs better than the exhaust-only system in controllingcarbon monoxide levels at occupied zones (0.5–1:5 m measured from =oor level) although more energy is consumed.? 2004 Elsevier Ltd. All rights reserved.

Keywords: Car park; Field measurement; Performance evaluation; Ventilation system

1. Introduction

To cope with the increasing number of registered privatecars, more multi-storey underground car parks were built inthe Hong Kong Special Administrative Region (HKSAR)as the land is expensive [1]. A recent address by the ChiefExecutive of the HKSAR indicated the policy of striving fora better environment. Complaints of poor air quality withinenclosed car parks have stirred up a hot issue in the com-munity since 1993 [2]. Previous studies conducted in 1996[3] revealed that the carbon monoxide (CO) levels in 25%of 103 car parks found in Hong Kong exceeded the WorldHealth Organisation (WHO) standard 1-h time-weightedaverage (TWA) of 25 ppm CO level [4]. This means thatit might bring adverse health impact to drivers, passengersand labour working in these premises.One of the aims of operating a ventilation system is to di-

lute the vehicular exhausts by introducing uncontaminatedair and extracting the mixture away from the enclosure.In many projects, calculation of the amount of outdoor airrequired relied on the “well-mixed” condition. However,mixing depends on the air=ow patterns where the locationsof supply points and extraction points of the ventilationsystem would be important. Note that supply and exhaust

∗ Corresponding author.E-mail address: bemychan@polyu.edu.hk (M.Y. Chan).

positions are located preferably at opposite sides. The ratioof volumetric =ow rate of high- to low-level exhaust inletswas speci5ed in the Greater London Council [5] to be 1:2.The regulation does not specify the details of such inlets,with tailpipe level of vehicles taken as ‘low level’, and levelabove occupants’ height as ‘high level’ to avoid nuisance.The supply outlets are placed at high level or level closeto the human breathing zone [6]. A possible explanation isfresh air can be delivered to occupants without mixing withvehicular exhaust [7].The level of CO depends on air distribution, traNc den-

sity and ventilation rate. Piston =ow ventilation in manycases might be better than conventional mixing ventilationin terms of contaminant control. With ideal piston =ow, theair change eNciency is 100% better than conventional mix-ing [8,9]. However, piston =ow is not always practical dueto congested traNc, space constraint resulting in supply andexhaust short-circuiting, and geometry (with sharp cornersto give stagnant position) of the car park. The theory usedin this paper assumes a mechanically ventilated, airtight en-closure where all the air enters or leaves via designated in-lets and exhaust ducts except in5ltration compensation forthe di7erence of supply and exhaust at the entrance.Six car parks were studied with performance of their ven-

tilation systems evaluated. These car parks were selectedbecause they are totally enclosed (roughly following theairtight assumption), underground and mechanically venti-lated. The number of cars entering and leaving the car park

0360-1323/$ - see front matter ? 2004 Elsevier Ltd. All rights reserved.doi:10.1016/j.buildenv.2003.10.009

636 M.Y. Chan, W.K. Chow / Building and Environment 39 (2004) 635–643

Nomenclature

A area of the car parkC(t) instantaneous carbon monoxide concentration of

a point at time t (m3 m−3)Ccp average carbon monoxide concentration of the

car park over a time period (m3 m−3)Ccp(∞) average carbon monoxide concentration of the

car park at steady state (m3 m−3)Ce average carbon monoxide concentration of ex-

haust duct over a time period (m3 m−3)Ce(t) carbon monoxide concentration of exhaust duct

at time t (m3 m−3)Ce(∞) carbon monoxide concentration of exhaust duct

during steady state (m3 m−3)Cej carbon monoxide concentration of the jth ex-

haust inlet (m3 m−3)Cs average carbon monoxide concentration of sup-

ply air over a time period (m3 m−3)Cs(t) carbon monoxide concentration of supply duct

at time t (m3 m−3)Cs(∞) carbon monoxide concentration of supply duct

during steady state (m3 m−3)Csi carbon monoxide concentration of the ith supply

outlet (m3 m−3)CT average carbon monoxide concentration over a

time period T (m3 m−3)Cz average carbon monoxide concentration of the

occupied zone over a time period (m3 m−3)G peak carbon monoxide generation rate (m3 s−1)m(t) volume of carbon monoxide that remained in the

car park at time t (m3)

m(∞) volume of carbon monoxide that remained in thecar park at steady state (m3)

n natural numberNop number of operating cars per hour (h−1)P total number of parking spacesQf outdoor air supply per =oor area (m3 s−1 m−2)Q total volumetric =ow rate of air (m3 s−1)Qej volumetric =ow rate of the jth exhaust inlet

(m3 s−1)Qinf volumetric =ow rate of outdoor air due to in5l-

tration at entrance (m3 s−1)Qsi volumetric =ow rate of the ith supply outlet

(m3 s−1)T time period (h)Qtj time step (h)V volume of the car park (m3)� ventilation eNciency (%)�co average carbon monoxide loading per hour

(m3 h−1)�co m−2 average carbon monoxide loading per hour per

=oor area (m3 h−1 m−2)� activity level of car park (%)� removal eNciency (%)�an nominal time constant of the ventilation system

(min)�cn turnover time of carbon monoxide (min)� average carbon monoxide removal e7ectiveness

(%)� speci5c =ow (ach−1)� local air quality index of occupied zone (%)

was counted. CO level was monitored at various parts of thesite including exhaust air duct, supply outlets, occupied ar-eas (including lift lobby, staircase and pedestrian route), andsome randomly selected areas. Results from measurementwere used to assess the ventilation system performance. Theindication of performance is e7ectiveness of pollutant re-moval and fresh air supply. The evaluation also focuses onthe spatial variation of pollutant and fresh air supply. Forcar parks with bigger =oor area such as Site B, of volume68; 000 m3 studied in this paper, the variation of pollutantlevels and fresh air quantity might cause elevated exposureto vitiated vehicular exhausts. A series of parameters wereused for assessment. The study eventually concludes on sug-gesting how car park ventilation systems can be assessedsystematically.

2. General descriptions of sites

Site A is a public housing estate with four parking levels.The total area of the car park is 2000 m2 and headroom 3 m,

which provides 133 parking spaces for private cars. Levels1 and 2 are naturally ventilated. Perforated bricks are laidalong one side for natural ventilation. Levels 3 and 4 aremechanically ventilated and built underground, which arealso the point of interest. The ventilation rate in terms of airchanges per hour at levels 3 and 4 is 0.55. The ventilationsystem is an exhaust-only system. The exhaust inlets centrelevel is 2:5 m from the 5nished =oor.Site B is a two-level underground car park sited at a large

commercial and shopping complex with an area of over16; 000 m2 for each level. The headroom is 2 m, which isconsidered to be low. There are two types of exhaust in-let, 0.5 and 1:8 m centre level from the =oor. It provides acapacity of 914 parking spaces, with 395 on the 5rst leveland 519 on the second level. Mechanical ventilation is pro-vided in both levels by a combined system with eight ex-haust fans and four supply fans serving each level. The sup-ply rate is less than the exhaust rate. There is in5ltration atthe main entrance and other possible air =ow paths. Outdoorair is directed to the supply outlets at the centre of the carpark and distributed to the periphery by means of di7usion.

M.Y. Chan, W.K. Chow / Building and Environment 39 (2004) 635–643 637

Table 1Summary of results of the six sites

Parameters Site A Site B Site C Site D Site E Site F

A (m2) 1454 34,080 9896 4000 2000 422V (m3) 4057 68,162 22,102 14,000 9500 1358P (number) 67 914 413 150 35 42Ventilation system Exhaust-only Combined Combined Exhaust-only Combined Exhaust-onlyNumber of levels 4 2 3 1 1 1Headroom (m) 3 2 2.2 2.5 4.75 3.2� (ach−1) 0.55 6.00 3.80 6.68 3.7 4.46Q (m3 s−1) 0.009 0.124 0.057 0.173 0.280 0.040Qf (m3 s−1 m−2) 0.0004 0.0033 0.0024 0.0065 0.0049 0.0040�co (m3 h−1) 13.3 42.3 2.9 3.4 0.3 0.2�co m−2 (m3 h−1 m−2) 0.009 1.240 0.293 0.860 0.140 0.578Ce (m3 m−3) 6 100 22 37 7 40Ccp (m3 m−3) 8 129 27 55 11 235Cs (m3 m−3) 0 2 3 5 3 4Cz (m3 m−3) 14 40 24 30 9 120�cn (min) 145 13 20 13 29 86�an (min) 109 10 16 9 16 13� (dimensionless) 75% 77% 78% 70% 56% 16%� (dimensionless) 7% 31% 17% 17% 36% 24%� (dimensionless) 75% 77% 81% 66% 69% 17%� (dimensionless) 33% 16% 26% 13% 12% 2%� (dimensionless) 43% 250% 92% 122% 80% 33%CO at exhaust duct (ppm) 6 100 22 37 7.2 40Average CO (ppm) 8 129 27 55 11 235CO at occupied zone (ppm) 14 40 24 30 9 120CO at supply duct (ppm) 0 2 3 4 3 4

The ventilation rate in terms of air changes per hour is 6during normal operation and 9 during congested hours. Inthe course of CO monitoring, the ventilation system wasoperating at 6 air changes per hour.Site C is an institutional building with three levels. The

total volume of the site is about 22; 000 m3. The total capac-ity is 413 parking spaces, with 137 on the 5rst level, 144 onthe second level and 132 on the third level. The ventilationsystem is a combined system with supply outlets and ex-haust inlets at opposite sides. The exhaust inlets are locatednear =oor level (0:1 m) and at 1:6 m, while supply outletsare located at occupancy level (1:7 m). The ventilation ratein terms of air changes per hour is 3.8.Site D is a single =oor commercial underground car park

with an area of 4000 m2, which provides 150 parking spaces.The enclosure is ventilated by an exhaust-only system. Thecentre level of exhaust inlets is 2:2 m from the ground, whichmeans that the vehicular exhausts would di7use throughthe breathing zone [6] before they are extracted away. Themake-up air is drawn through the entrance by virtue of nat-ural draughting. The ventilation rate in terms of air changesper hour is 6.Site E is a hotel basement car park, which serves exclu-

sively for the tenants and sta7. The total area is 2000 m2

with high headroom of 4:75 m. The number of parkingspaces is 35. The enclosure is ventilated by four exhaust fansand two supply fans. The supply outlets and exhaust inletsare 2.2 and 1:8 m from =oor level respectively. The sup-ply and exhaust grilles are arranged alternately in an array.

The ventilation rate in terms of air changes per hour (ach)is 4, less than 6 ach as speci5ed in local codes.Site F is an underground car park with an area of 422 m2

and 42 parking spaces. The traNc is heavily congested onSunday mornings. The car park is of an attendant-parkingoperation type. The sta7 of the car park is in charge of theoperations in the reservoir space, directing the movementof incoming cars. The sta7 also operate the parking in thestorage area. The storage of vehicles is closely packed to-gether with no space left around. The ventilation rate interms of air changes per hour is 4.46. The ventilation systemis composed of two axial extraction fans at one end at 3 mabove ground level. Fresh air is induced by means of natu-ral draught at the entrance, implying that the assumption ofairtight (without in5ltration) at building crack holds.The details of Sites A–F are shown in Table 1. The plans

of Sites A–F are shown in Figs. 1–6.

Fig. 1. Site A plan view.

638 M.Y. Chan, W.K. Chow / Building and Environment 39 (2004) 635–643

Fig. 2. Site B plan view.

Fig. 3. Site C plan view.

Fig. 4. Site D plan view.

3. Carbon monoxide balancing

In a car park with ventilation rate Q (m3 s−1) and COgeneration rate of G (m3 s−1), mass balancing for perfect

mixing gives the CO volume m (m3) and concentration atexhaust duct Ce (m3 m−3) at time t without natural in5ltra-tion:

G − QCe(t) = dm(t)dt

: (1)

At non-peak hours, Ce(t) tends to be a steady-state valueCe(∞), for zero rate of change of m(t):

Ce(∞) =GQ: (2)

Putting in Eq. (2), the volume of CO remained in the carpark at steady-state, m(∞) can be estimated as

m(∞) = Q∫ ∞

0[Ce(∞)− Ce(t)] dt: (3)

Steady state condition: The spatial average of CO levelat steady state in the car park Ccp(∞) is the theoreticalconcentration under perfect mixing condition at steady state

Ccp(∞) =m(∞)V

(4)

or

Ccp(∞) =QV

∫ ∞

0[Ce(∞)− Ce(t)] dt: (5)

Note that a mixing factor might be added for incompletemixing.The average concentration Ccp(∞) will be equal to the

exhaust concentration Ce(∞) if the ventilation is of conven-tional mixing type, under perfect mixing condition, leadingto

Ccp(∞) =GQ: (6)

4. Key parameters

The following parameters were measured in this 5eldstudy:

• Exposure to CO: The time prolonged of the occupantsinside the car park and concentrations prevalent are thetwo major elements in assessing the exposure to pollu-tants. It is common to describe the two components to-gether as time-weighted average [10]. The levels of COprevalent in the car parks may =uctuate with time and soaveraging is necessary for a better description of the con-centration. The resulting exposure level to CO for a timeperiod T; CT , i.e. the T hour time-weighted average, canbe expressed in terms of the average CO concentration Ciat the ith hour [11]:

CT =1T

n∑i=1

Ci: (7)

• Exhaust and supply concentration: For an enclosedcar park with several supply outlets and exhaust inlets

M.Y. Chan, W.K. Chow / Building and Environment 39 (2004) 635–643 639

Fig. 5. Site E plan view.

Fig. 6. Site F plan view.

of volumetric =ow rates Qsi and Qej at the ith outlet andjth inlet, respectively, the =ow is steady and change ofair density is negligible. The average supply (Cs) andexhaust (Ce) concentration over a time period of n supplygrilles are de5ned in terms of the CO concentration Cs at

the ith supply [12]:

Cs =∑n

i=1 QsiCsi∑ni=1 Qsi

: (8)

Similarly, Ce is given in terms of the CO concentrationCej at the jth exhaust inlet:

Ce =

∑nj=1 QejCej∑nj=1 Qej

: (9)

Neglecting the change of air density, conservation of vol-umetric =ow rate in terms of the volumetric =ow rate Qinfof outdoor air due to in5ltration at the entrance givesn∑i=1

Qsi + Qinf =n∑j=1

Qej: (10)

Note that i stands for the ith supply and j for the jth ex-haust. Usually, the exhaust volume is greater than the sup-ply volume. The di7erence is compensated by in5ltrationat the entrance.For simplicity,

Q =n∑i=1

Qsi + Qinf =n∑j=1

Qej: (11)

• Speci5c 6ow: Ventilation rate has always been ex-pressed in terms of ‘air changes per hour’, giving theerroneous impression that air is completely replaced atthe given air change rate. But for the design practice inHong Kong, while fresh air can be supplied at the given

640 M.Y. Chan, W.K. Chow / Building and Environment 39 (2004) 635–643

‘air change rate’, the ‘replaced air’ is normally a mixtureof ‘stale’ and ‘fresh’ air. That is why there is a degreeof mixing describing the mixing proportion of stale andfresh air. For this reason, ‘speci5c =ow’ (�) is useful fora car park of volume V :

�=3600× Q

V: (12)

• Car park CO loading per hour: The quantity of CO dis-charged from vehicles is a key factor in determining theventilation rate. The ideal case is to count the total num-ber of cars operating and measure the individual exhaustquantity from each vehicle. However, it is almost impos-sible to carry out such measurement and so monitoring theaverage concentration of CO in the exhaust air stream andvolumetric =ow rate of all exhaust inlets was proposed(with in5ltration and ex5ltration neglected). Averagingover time will give the average CO loading per hour. Thecar park CO loading (�co) per hour can be given in termsof the time step (Qtj) of each measurement as

�co =

∑nj=1 QejCejQtj

T: (13)

The total sampling time T is given by

T =n∑j=1

Qtj: (14)

The monitoring is normally expected to cover the con-gested period and at least for a working shift of 8 h.

• Activity level: Activity level (�) is the ratio of total num-ber of operating cars observed to total capacity of the carpark in an hour. There are automatic counters at the en-trance and exit in these car parks and so the number ofoperating cars Nop (includes those entering or leaving thecar park) can be recorded. The total capacity of the carpark is the total number of parking spaces (P) provided:

� =NopP; (15)

• Nominal time constant of ventilation system: Time con-stant is the time interval from the time the air is present ata particular location to the time the air leaves the car park.A long nominal time constant is normally associated withpoor indoor air quality. The overall average time constantin a car park is called the ‘nominal time constant’ (�an) ofthe whole car park. It is also the inverse of the speci5c=ow and hence is derivable from the ventilation rate andthe volume of the car park [12]. It is independent of theinternal =ow pattern or pollutant properties. The unit ofnominal time constant interpreted here is minutes:

�an =V

Q × 60: (16)

• Turnover time of CO: It is similar to that of nominaltime constant of air, but it represents the turnover time(�cn) of CO on an average [12]. The unit of turnover time

interpreted here is minutes. The mathematical expressionis given by

�cn =m(∞)G × 60

: (17)

• CO ventilation e9ciency: The ventilation eNciency (�)re=ects the performance of a system with regard to theaverage ability to remove pollutants. It is based on theaverage car park concentration, supply concentration andaverage exhaust concentration over a time period [9]. Itis given by the equation

� =Ce − CsCcp − Cs : (18)

If the supply concentration is near zero, or the exhaustconcentration and average concentration are much higherthan the supply concentration, then Eq. (18) can be ap-proximated to

� =CeCcp: (19)

The ventilation eNciency at steady state is

�(∞) =Ce(∞)Ccp(∞)

: (20)

From Eqs. (2) and (4)

�(∞) =�an�cn: (21)

• CO removal e:ectiveness: CO removal e7ectiveness (�)is the description of movement and dilution of CO withina car park. Indices of CO removal e7ectiveness are depen-dent on both the characteristics of air=ow and the char-acteristics of the CO emission [9]. It is given by the ratioof average concentration of CO at the exhaust and thezone average over a time period. For conventional mix-ing ventilation, perfect mixing will give 100% e7ective-ness. Short-circuiting will give zone average concentra-tion greater than the exhaust concentration. The CO re-moval e7ectiveness will range between zero and unity.For piston =ow ventilation, the CO removal e7ectivenesswill be greater than unity. The mathematical expressionis given by

�=CeCcp: (22)

This is the average e7ectiveness over a time period.• CO removal e9ciency: The CO removal eNciency (�)is a normalised value of the CO removal e7ectiveness. Ifthe ventilation system is of a conventional mixing type,complete mixing of pollutants within the car park is 0.5.The de5nition of mixing ventilation is always explainedin terms of the age of air. Complete mixing is a conditionwhere the local mean age of air throughout the enclo-sure is the same and equal to the nominal time constant.

M.Y. Chan, W.K. Chow / Building and Environment 39 (2004) 635–643 641

It can also be explained in terms of the local =ow rate.If complete mixing occurs, the local =ow rate throughoutthe enclosure is the same and equal to the aggregate =owrate [12].The supply air and enclosure air are mixed by the actionsof the supply momentum and buoyancy. Under this phe-nomenon, the supply air is used to dilute the concentrationof CO and other pollutants in the car park. The mixtureof air and CO or other pollutants is then extracted awayvia the exhaust duct. The above operation occurs as nu-merous =ow 5elds in the enclosure, but the mixing ratioof air and pollutant varies from one location to another.Imperfect mixing is due to short-circuiting of supply andexhaust. Short-circuiting gives a value of � between 0 and0.5. Piston =ow ventilation gives a value between 0.5 andunity. Piston =ow ventilation is a unidirectional =ow ofair in which supply air propels the pollutant ahead of itlike a front. The action is analogous to a piston inside acylinder, ‘sweeping’ the air to the outlet. That is why itis called “piston =ow”. The two-air side partitioned, withno or very few mixing is the basic requirement. In orderto achieve this criterion, the air turbulence must be re-duced to a minimum so that the dispersion of pollutant isminimised:

�=Ce − CminCmax − Cmin ; (23)

where Cmax and Cmin are the maximum and minimumvalues of CO levels inside the car park, respectively.

• Local air quality index at occupied zone: The local airquality index (�) at occupied zone is the ratio betweenthe average concentration of CO at the exhaust (Ce) andat the occupied zone (Cz) over a time period, e.g. shro7oNce, lifts lobby, pedestrian walkway, etc.,

�=CeCz: (24)

5. Results

The results on the measured key parameters are sum-marised in Table 1. The following are observed.Site A was not heavily utilised, so the surveyed activity

level (�) was only 7.4%. The 5gure was averaged from an8-h survey. The average CO loading (�co) was 13:3 m3 h−1.The normal rate is expected to be 15–20% as speci5ed inthe literature such as ASHRAE Handbook [13]. Both themaximum concentration and 8-h time-weighted average didnot exceed the values speci5ed by WHO [4]. The CO re-moval e7ectiveness (�) was 75%, which means that the av-erage concentration was higher than the exhaust concentra-tion (Ce). Imperfect mixing was the reason for this phe-nomenon. The local air quality index (�) at the occupiedzone was 43%, which was worse than the average. The re-moval eNciency (�) was 0.33, which implies conventionalmixing.

Site B was heavily utilised throughout the survey. Thesurveyed activity level (�) was 31%, which means therewere over 280 cars operating in the car park per hour. TheCO loading (�co) was 42:29 m3 h−1. The average loadingof CO per =oor area (�co m−2) was also the highest amongthe six sites. The fresh air supplied (Qf ) to the site per =oorarea was ranked fourth in descending order. It is the mainreason why CO level (Ccp) was the highest, with 78 ppm for8 h. Referring to the WHO standard [4], 50 ppm is the max-imum permissible exposure level for the 8-h time-weightedaverage.Site C was heavily congested in the mornings and

evenings. The usage other than these periods was relativelylow. The 8-h time-weighted average was suppressed to21:65 ppm due to the above reason. The maximum levelin the morning reached a peak of around 200 ppm. Theoriginal design concept was a piston =ow system, but thederivation of removal eNciency (�) had shown that it is aconventional mixing system. Removal eNciency is between0% and 100% (Eq. (23)).For Site D, peak hours were found in the evenings and dur-

ing holidays. The 8-h time-weighted average was 55 ppm,which slightly exceeded the recommended level [4]. Theventilation system is an exhaust-only system. At some par-ticular locations, the CO levels (e.g. non-pedestrian route)were extremely high because of insuNcient outdoor air sup-ply. The average CO removal e7ectiveness (�) was 66.7%.The e7ectiveness was said to be fairly acceptable.The surveyed activity level (�) of Site E was 36.2%,

which was the highest. However, the 8-h time-weighted av-erage was 24 ppm. It was due to suNcient outdoor supply,good supply air distribution and high headroom. The out-door air supplied per unit =oor area (Qf ) was 0:0049 m3 s−1,which was the second highest. The supply outlets are evenlydistributed along the walkway for users and the lift lobby.Outdoor air should be delivered to the occupied areas di-rectly.Site F is of the attendant operated type and it was heavily

congested. Note that attendant operated type is more eN-cient for smaller car parks in terms of parking and de-parkingtime required. The density of parking was so high that thearea per parking space was less than the recommended valueof the Australian Standard, i.e. 23 m2 per space [14]. Thearea per parking space is only 10 m2. There is no localstandard governing the parking space requirement in HongKong. Momentarily, high levels of CO (say up to 235 ppm)were recorded at the areas near the exhaust outlet. The sur-veyed activity level (�) was 24.4% averaged over 8 h, butduring some occasional periods, the activity level (�) wasover 100% on Sunday morning. The activity level is de-5ned as the percentage of turnover per hour. It is possi-ble to have 5gures greater than 100%, implying that somecars stay in the car park for less than an hour. The localair quality index (�) was 35%, being the worst one of thesix car parks. It is because the exit staircase is close to theparking area and exhaust outlet without isolation. Fresh air

642 M.Y. Chan, W.K. Chow / Building and Environment 39 (2004) 635–643

supply at the main entrance was heavily degraded by carexhaust at the entrance queue.

6. Discussion

The measured 8-h time-weighted average of CO level atSite B was 78 ppm, the highest among the six sites. It wasdue to the high loading of CO emission from cars. The emis-sions from vehicles are close to the occupied areas. Duringthe monitoring of CO at Site B, the ventilation system wasoperated at normal ventilation rate. The amount of fresh airsupplied to unit =oor area was nearly the least. The dilutionof CO was not as good as the others.For CO removal e7ectiveness (�), the values lie between

17.2% and 80.5%. It is a ratio of exhaust concentration toaverage CO levels, which compares the level of CO preva-lent at average condition. All of them are less than unity.It demonstrates the imperfect mixing of air and CO at allsites (see Table 1). However, some of them are very closeto unity. The removal e7ectiveness is satisfactory at thesesites.Normalised removal eNciency (�) examines the type of

ventilation system. For conventional mixing ventilation, thevalues lie between 0 and 0.5. For piston =ow ventilationsystem, the values lie between 0.5 and unity. The normalisedremoval eNciency (�) of the six sites falls between 0 and0.5. The type of ventilation is conventional mixing.The nominal time constant (�an) of the six sites ranged

from 9 to 110 min. This parameter shows the average du-ration of stay of outdoor air from entering to leaving thecar park. The ideal case would be as short as possible. Asspeci5ed in ASHRAE Application Handbook [13], the rec-ommended ventilation rate is 6 ach, corresponding to a timeconstant of 10 min. However, it does not absolutely guaran-tee a good air quality even if the time constant is very short.Another parameter, turnover time (�cn) of CO will determinethe average duration of stay of CO inside the car park. If theventilation eNciency of a system is invariable, a short timeconstant will result in a short turnover time. A better indoorair quality can be expected.If the supply concentration (Cs) is zero, or the exhaust

concentration (Ce) and car park average concentration aremuch higher than the supply concentration, ventilation eN-ciency (�) can be reduced to CO removal e7ectiveness (�)at steady-state condition. Removal eNciency (�) shows theability of the system to remove pollutants. CO removal ef-fectiveness (�) shows the control of average CO level incomparison to the exhaust level. If the mixing is perfect,and the system is conventional mixing, � will also be equalto � whatever the supply concentration or the di7erence is.The resulting value is 100%.It is almost impossible to measure the emission and CO

left in the car park at steady state accurately. The only wayto obtain the turnover time of CO is to take the ratio as

shown in Eq. (17) under stipulated conditions. It is a derivedquantity and not an independent parameter.

7. Conclusion

The removal e7ectiveness (�) had shown that the sixsites employed conventional mixing system. It is very diN-cult to obtain piston =ow ventilation even when the supplyand exhaust ends are separate at opposite sides with care-ful arrangement. The undesired mixing e7ect was due to carmovement, momentum force of supply air and buoyancyforce of car exhausts. Although the removal eNciency (�) ofpiston =ow is twice that of conventional mixing, elevationof concentration may cause high levels of pollutant near theexhaust inlet. If the occupied areas (lift lobby, pedestrianroute) are close to the exhaust inlets, piston =ow ventila-tion will cause adverse health e7ects to the occupants. Thiscondition will end up with a local air quality index (�) ofCO equal to unity or less than unity. A good distribution ofsupply air will normally have an even local air quality indexgreater than unity at all areas. Surplus supply of outdoor airat occupied zone is a good solution to improve the local airquality index. Drivers and car park attendants spend mostof their time in these areas, normally taking a few minutesto park the cars. If the concentration of pollutants in theseareas can be suppressed down to a relatively low level, theexposure to vitiated exhaust can be greatly reduced.The local air quality indices (�) for the combined sys-

tems at Sites B, C and E were 250%, 91.7% and 80%, re-spectively. These show that a relatively low level of CO wasmaintained at the occupied zone, indicating that air qualitywas improved.Various literature [13,15] had speci5ed the outdoor air

supply for each operating car in underground car parks. Themost often seen 5gure is 0:236 m3 s−1 per operating carwith an average simultaneous usage of 3.5% and 5% peakof the total capacity of the car park. This is equivalent to a5gure of 0.00826 and 0:0118 m3 s−1 per parking space foraverage and peak, respectively. All sites satisfy the require-ment except Site A.The removal eNciency (�) indicates that the concentra-

tion gradient within the car park is quite large. It might bedue to the stagnant zones created and the degree of mixing,which varied a lot with di7erent locations. Relatively highlevel of CO was prevalent at occupied areas. At Sites A andF, the local air quality indices (�) were 43% and 33%, re-spectively.Piston =ow ventilation had been proved to be more supe-

rior to conventional mixing provided that the occupied areasare not close to the exhaust inlets. It is concluded that it isdiNcult to achieve piston =ow ventilation practically undervarious constraints. Mixing ventilation becomes the only al-ternative. The combination of mixing ventilation and localexhaust strategy at tailpipe level near parking stall wouldbe the preference. The local exhaust inlets can minimise the

M.Y. Chan, W.K. Chow / Building and Environment 39 (2004) 635–643 643

emission strength so that the burden of dilution ventilationsystem can be reduced.

Acknowledgements

The authors wish to thank the cooperation of the car parkmanagement teams from various organisations.

References

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[2] Hong Kong Consumer Council. Choice, February 1993. p. 14–20(in Chinese).

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